1. Field of the Invention
The invention relates to an apparatus for demodulating an n-phase shift keying modulated wave carrying a digital signal.
2. Related Background Art
As a modulation system for transmitting digital data, an amplitude shift keying modulation (ASK) system, a phase shift keying modulation (PSK) system, a frequency shift keying modulation (FSK) system, and the like are known in the art.
When considering a multiplex transmission of a frequency modulated analog video signal and an audio signal, the phase shift keying modulations among the modulation systems are widely used in view of their features that a band limitation upon transmission can be performed, an interference to the other signals hardly occurs, a signal-to-noise ratio is good, and the like. In the PSK modulation systems, quadrature phase shift keying modulation (QPSK) system has a highest efficiency. According to the QPSK modulation wave, since a phase amplitude change is steep, a delay detecting circuit in a decoding apparatus of recording/reproduction modulation waves can be easily realized as a digital circuit. The QPSK modulation system for recording and reproducing a digital signal is, therefore, used in an information recording and reproducing apparatus disclosed in, for example, Japanese Patent Application No. 5-291071.
The QPSK modulation system disclosed in the Japanese Patent Application No. 5-291071 will now be described.
It is now assumed that base band data to be transmitted is as follows. That is, base band data "00 01 10 11 11 00 10 01" is assumed to be transmitted. Redundancy bits which indicate various information and is used for correction of a bit error and sync data bits for synchronization are generally included in the base band data. There is also a case where some ID data bits are further included.
The base band data is not modulated as it is, but is subjected to an addition of modulo 4 accumulatively by processing two bits as one unit for the sake of convenience of demodulation, thereby obtaining the following data. Consequently, 00 01 00 11 00 00 10 00 is obtained. If this data sequence is expressed as an (I, Q) data sequence and odd-number designated bits are expressed as I bits and even-number designated bits are expressed as Q bits, the original base band data is converted to the I bit sequence and the Q bit sequence, respectively. The I bit sequence and Q bit sequence, namely, I data and Q data are respectively supplied to an I input and a Q input in the QPSK modulator shown in FIG. 1.
A carrier signal fc (=2.88 MHz) is supplied to a carrier input terminal. The I data and Q data are transmitted through low pass filters 1 and 2, and are supplied to modulation signal input terminals of modulators 3 and 4, respectively. The carrier signal is supplied as it is to a modulated signal input terminal of the modulator 3. The carrier signal is delayed in phase by .pi./2 by a delay circuit 5, and the delayed signal is subsequently supplied to a modulated signal input terminal of the modulator 4. For example, when the input data is equal to "0", the modulators 3 and 4 allow the carrier signal to pass as it is. When the input data is equal to "1", the modulators 3 and 4 invert the phase of the carrier signal. Outputs of the modulators 3 and 4 are added by an adder 6 and a QPSK modulated wave is generated.
As shown in FIG. 2, the phase of the QPSK modulation wave obtained as mentioned above is delayed by -.pi./4 with respect to the original carrier phase when (I, Q) are (0, 0). When (I, Q) are (1, 0), it is delayed by -3.pi./4. When (I, Q) are (1, 1), it is delayed by -5.pi./4. When (I, Q) are (0, 1), it is delayed by -7.pi./4. In other words, the carrier phases .pi./4, 3.pi./4, 5.pi./4, and 7.pi./4 correspond to the (I, Q) data, respectively.
The parts (a) and (b) of FIG. 3 respectively show an I data waveform and a Q data waveform which are transmitted through the low pass filters 1 and 2. The part (c) of FIG. 3 shows a waveform of the QPSK output. Two bits of the (I, Q) data are referred to as one symbol, a transfer rate of a symbol train is set to 288 ksymbols/sec, and this corresponds to ten waves of the carrier having the frequency 2.88 MHz. The QPSK modulation wave which is obtained in such a manner as mentioned above is recorded to a recording medium or is transmitted by a proper transmission medium such as an RF channel or the like.
It is known that the modulator shown in FIG. 1 is realized by a method whereby the carrier waveforms of four phases, namely, the QPSK modulation waveform data is previously stored as a data table in a ROM and the carrier waveform data of the phase corresponding to the (I, Q) data that is derived on the basis of the original base band data is read out from the ROM and then processed by a D/A conversion. A circuit construction to execute the method is disclosed in FIG. 5 of Japanese reference 5-291071 and the corresponding description of the specification of Japanese Patent Application No. 5-291071.
In the case of demodulating the QPSK modulation wave, for example, a system for delaying and detecting the QPSK modulation wave and demodulating it is used. In the delay detecting system, although there is a possibility that an error occurs in the delay detection due to a waveform distortion of the QPSK modulated wave or a variation in rotational speed of an optical disk or the like, what is called an error rate has to be reduced.
To reduce the error rate in the delay detection of the QPSK modulated wave, a method of raising a sampling frequency in the delay detection is conceivable. The use of this method would result in an enlargement in scale of the circuit for the delay detection and an increase in electric power consumption.